Dark elves were elves, but not like high elves. They were beautiful in their own way, too, but it was a cold, haughty beauty, one that in the end repelled. The beauty of the dark elves was said to be almost as chill as Deathwing's voice, if not colder. They were lesser beings with minute lifespans and quick to rashness, plunging into what was believed too great a risk. In their folly, they brought demons.DotD 160-161

Retconned to night elves?

This article or section includes speculation, observations or opinions possibly supported by lore or by Blizzard officials. It should not be taken as representing official lore.

Early concept maps show "dark elf lands" in northeastern Kalimdor near Mount Hyjal. A race is said to have spoken a fourth elven dialect Elven on Mount Hyjal.CoH 67LoM 17 Perhaps this is the dark elves.

Most evidence points to the mysterious dark elves as being an early developmental form of the night elves. It is hinted that the dark elves, possibly along with others (the book says, "others came, lesser forms, minute life spans"),DotD 161 had something to do with releasing the Burning Legion on Azeroth. According to interviews and references made in The Art of Warcraft and on the The Making of World of Warcraft DVD, night elves were based off the dark elves of other fantasy settings (especially the drow from D&D), but their personality and culture were made to be the direct opposite of the culture and personality of dark elves. A story nearly identical to the one told about the dark elves in Day of the Dragon is later related to Khadgar in The Last Guardian by Medivh, where he gives these ancient magic-users a more familiar name: Kaldorei. It seems most likely, therefore, that the "dark elves" are in fact the night elves under an earlier name.

Dark elves may be a name given to night elves through the lens of history and memory of the cruel and corrupt Highborne.

Trivia

Dökkálfar (dark elves) are known as a class of elves living underground in Old Norse mythology. They are subterranean creatures like some dwarves.

In Tolkien's universe, the dark elves (Moriquendi) are elves that did not join the Great Journey over the sea and behold the light of the two trees of Valinor. They are more similar to night elves in that they are not evil.

Dark elves, or drow, are a generally evil, dark-skinned subrace of elves in the Dungeons & Dragons fantasy roleplaying game. They were forced underground after a great war between elves.

Dunmer (dark elves) are a class of elves native to the province of Morrowind in The Elder Scrolls series of games. They have a natural resistance to fire, and are considered to be inferior to the high elves, or Altmer. They have a generally pallid complexion, as well as red or black eyes.